Evidence is accumulating that moderate levels of sustained local hyperthermia, alone or in conjunction with other methods of therapy, is useful in the management of malignant tumors. A scaled-down labortory prototype of a computer-controlled insonation system for deposition of ultrasonic energy within a predetermined volume of tissue, by scanning the region in a programmed manner, has been demonstrated to produce and maintain the prescribed level of hyperthermia throughout the target volume for the desired duration. Design and fabrication of a full-scale model is in progress. Prior to its tests on spontaneous tumors in domestic animals, optimization of the design and computer programs, through its use in large experimental animals, is proposed.